Many displays are difficult to read when viewed in sunlight. Fortunately, optical filters offer a low-cost solution to this problem.
To specify a filter, the lighting conditions a display will be used in and how the eye responds to these conditions must be known. While different types of displays (for instance, CRTs and LEDs) generally have vastly different design considerations, there are common points. In general, a filter should maximize the contrast between display and background. In high ambient light, both display brightness and color are important.
Color is important because the eye is more sensitive to some colors than others. Therefore, even though two light sources may have the same radiant flux, one may appear brighter than another. Eye sensitivity peaks at a wavelength of about 555 nm, which is a yellow-green color. As a result, it is difficult to produce high contrast for green displays through filtering.
Color contrast is usually described by two quantities: chromatic distance and chrominance index. These characteristics are determined with a chromaticity diagram. Data points for both the background color and the color of the emitted light are plotted on the diagram. The theoretical chromatic distance between the points is then calculated with an equation that takes the mixing of emitted and reflected light into account.
The smallest discernible color difference is a dimensionless number called threshold chrominance, which has a value of about 0.00384. However, to easily differentiate between two colors, a difference of about 0.027 is recommended. This value is called the unitary color difference.
Chrominance index (IDC) is the ratio of the display's chrominance difference to the unitary color difference. Knowledge of a display's IDC is important for two reasons. First, an IDC of one implies that the display's color difference is just large enough to be easily recognized. Secondly, the IDC, in combination with a similar index for brightness, is a measure of visibility in high ambient-light conditions.
Luminance contrast compares the display brightness with its background. For LEDs, display brightness includes the ambient light reflected off the element itself. For a filtered LED display, a contrast ratio, called the luminance contrast ratio Cr can be defined as: